(a) Field of the Invention
The present invention relates to a quadrature demodulator used for a high-speed wireless communication system. More specifically, the present invention relates to a method for detecting and correcting amplitude and phase imbalances between I and Q components of complex signals generated by a quadrature demodulator
(b) Description of the Related Art
In a high-speed wireless communication system, the demodulator receives signals through an antenna, a radio frequency (RF) unit, and an intermediate frequency (IF) unit. The RF unit and IF unit convert the high frequency to a low frequency and amplifies the signals to generate desired signals.
The RF unit and the IF unit utilize various analog elements including mixers and amplifiers that satisfy specifications within a certain range, but the specifications have some limits, and input signals are distorted by incompleteness of isolation and phase orthogonal properties between the elements.
Distortion factors include amplitude and phase imbalances between in-phase (I) and quadrature-phase (Q) components at the quadrature demodulator, in, e.g., QPSK and/or QAM demodulators. The amplitude and phase imbalances between I-phase and Q-phase components cause degradation of the performance of the demodulator in the modem for recovering signals. Therefore, methods for removing the amplitude and phase imbalances between the I and Q components are required.
Regarding removal of the amplitude and phase imbalances between I and Q components, several methods using an RF direct conversion receiver are disclosed. The U.S. Pat. No. 6,044,112 entitled “Method and apparatus for correcting amplitude and phase imbalances in demodulators” by Joshua L. Koslov dated Mar. 28, 2000 discloses a method for providing several complex adders and multipliers as well as a counter to the demodulator to correct gain and phase imbalances. However, the method by Koslov increases implementation complexity by using complex multipliers and is sensitive to noise because it utilizes a simple counter at the demodulator. Further, since this method sets the amplitude and phase imbalances between I and Q components according to counter step, its response speed depends on the increase and decrease step size of the count. The method does not reflect the actual received signal, but depends on counter step size.
In addition, the U.S. Pat. No. 5,949,821 entitled “Method and apparatus for correcting phase and gain imbalances between in-phase(I) and quadrature (Q) components of a received signal based on a determination of peak amplitudes” by Shahriar Emami discloses a method for detecting amplitude peak values of demodulated I and Q components, and correcting the amplitude and phase imbalances between I and Q components using the peak values. The patent sets one of the I and Q components as a reference component and another one as an imbalance component, finds the amplitude peak value of each component, and finds the phase imbalance using sine functions. It uses arcsine functions to determine the phase imbalances between I and Q components, but this application of the arcsine functions increases digital circuit complexity or degrades the system's performance.
The above-noted patents have failed to solve the problems of circuit implementation complexity and noise sensitivity, so an improved method of detecting and correcting the amplitude and phase imbalances between I and Q components is now required.